Rocks of Upper Rockridge III: Blueschist

At the uppermost top of Upper Rockridge is Contra Costa Road, where amid the fine homes and gardens is this knocker of real blueschist.

Knockers are the California geologist’s nickname for blocks of resistant rock in Franciscan mélange, the smorgasbord of rock types (chert, basalt, greenstone, serpentinite, etc.) bound by shaly matrix that is common throughout the Coast Ranges. Knockers are too big to be called boulders but too small to be mapped. Anyway, this knocker is a tough stone of a deep indigo color from the high-pressure mineral glaucophane, which if you remember your Greek simply means “blue in appearance.” I couldn’t resist taking home a chip.

Unlike the garnet-mica blueschist of Joaquin Miller Park, this outcrop is almost monomineralic except for some white veins, probably quartz. It gleams like leather in the magnifier, with intricate crenulations and understated foliation—not a real schisty schist, but layered enough to qualify. I’m in love with it.

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This entry was posted on 24 October 2010 at 8:56 pm and is filed under Franciscan rocks, Oakland blueschist. You can follow any responses to this entry through the RSS 2.0 feed.
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I want you to come and live with me and do a running commentary on the geology in my life. I am SO grateful whenever anyone can tell me something about the geology of a area. Like why are those lone buttes on the way to Chico? and why is the dirt red in Redbluff and Redding and nowhere else? What are those yellowish grayish rocks in the clay in my back yard that shatter so easily? You get the picture. You would have to move to San Mateo…

Mary Beth:
Those lone buttes on the way to Chico are the “Sutter Buttes” – An old eroded volcano. It is the southern point of the Cascade Range, which includes Shasta and Lassen. These mountains extend north to Oregon and Washington State.